Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common hereditary diseases, causing loss of renal function that leads to dialysis treatment in some 1800 new patients a year. The disease, arising from mutations in the PKD 1 gene encoding polycystin-1, is characterized by the presence of multiple renal cysts. Expansion of these cysts correlates closely with loss of renal function and is driven by transepithelial fluid secretion. The objective of this KO8 proposal is to understand why loss of polyeystin-1 results in fluid secretion and cyst expansion. ATP is a potent and universal stimulus for fluid secretion and might therefore play a role in ADPKD: All the components required for ATP-stimulated chloride secretion are resident in the cyst environment. ATP acts through heterotrimeric G proteins (Glphabetagamma) to cause an initial burst of calcium release from the endoplasmic reticulum (ER), which in turn triggers a prolonged phase of calcium entry from outside the cell (agonist induced calcium entry or ACE). ACE is also triggered by ER store-independent pathways linked to Galphabetagamma. Increased ATP-stimulated chloride secretion could arise from loss of polycystin-1 through disruption of its known effects upon ACE-type channels and Galphabetagamma. In preliminary experiments it was demonstrated that expression of the isolated C-terminal 193 amino acids of polycystin-1 (sIg-PKD193 fusion protein) augmented ATP-stimulated chloride secretion through up-regulation of store independent ACE in a cortical collecting duct cell line. Over-expressed polycystin-1 had the opposite effect, abbreviating the cell calcium response to ATP, suggesting that the slg-PKD193 fusion protein effect was the result of dominant negative inhibition of endogenous polycystin-1. It is hypothesized that: Polyeystin-1 down-regulates ATPstimulated chloride secretion through attenuation of store-independent agonist-induced calcium entry, and acts via modulation of heterotrimerie G protein coupled pathways. The experiments in Specific Aim #1 will define the effects of polycystin-1 upon the ER store dependent and independent components of ACE, and correlate these effects with the regulation of chloride channel activity. This will be done through over-expression and inhibition of polycystin-1. Specific aim #2 will identify the mechanism of the effect of polycystin-1 upon ACE, thereby providing a direct link between genetic lesion and fluid secretion. Dr. Michael Sutters has dedicated his career to becoming both a clinician and a scientist. He will be the principle investigator in this KO8 application. The long-term aim of his work is to facilitate the design of new therapeutic strategies to control disease progression through delineation of the mechanism of cyst expansion in ADPKD